The most widely used electrophotographic print apparatus employs a movable photoconductor which is selectively exposed by a source of optical energy. While such electrophotographic printers have been widely accepted and produce excellent print quality at reasonable cost, continued efforts are being directed to increase their performance and further reduce their cost. However, photoconductor-based printers will continue to exhibit certain problems which inherently arise from the use of a photoconductor. Among those are the cost of the photoconductor, photoconductor wear; and photoconductor sensitivity to light requiring continual shielding. Further, when an image is fully developed on the photoconductor, a transfer action must occur to enable removal of the toner to a media sheet.
Recently, a new class of electrostatic printers has been developed which requires no photoconductor and avoids many problems inherent with the use of the photoconductor. That class of printers comprise "toner projection printers" which include a system of electrodes for controlling direct deposition of charged toner particles on a media sheet without an intervening photoreceptor or photoconductive device. Typically each electrode includes a conductive electrode ring surrounding a hole in an insulating substrate. On one side of the substrate is a developer module which includes a developer roll and a supply of charged dry toner particles.
For a system employing negatively charged toner particles, when an electrode ring is driven positive with respect to the developer roll, the toner particles are attracted to the electrode ring and some pass through the hole. On the opposite side of the insulating substrate is a media sheet which rests on a conductive platen. The platen is biased to a voltage that is more positive than the electrode ring so that toner particles are attracted to the paper/platen combination. The toner particles that pass through the hole all exhibit a like charge. Thus, those particles repulsively interact during their travel to the media sheet and the result is some toner "spreading" at the point of deposition. Also, previously deposited toner exhibits a like charge which further adds to toner spreading. As a result, the edge definition of the deposited toner dots becomes less sharp.
Toner that is attracted to the electrode ring but does not path through the aperture, collects around the aperture and must be removed periodically. This is accomplished by reversing the potential between the electrode ring and the developer roll to pull such toner deposits away from the insulating substrate and electrode ring and back to the developer roll.
U.S. Pat. No. 5,036,341 to Larson et al. describes a toner projection printer wherein the print control matrix comprises two layers of parallel wires in each of two layers. The two layers are orthogonal and are disposed parallel to the plane of a media sheet upon which the toner is to be developed. The wires in each layer are arranged in the form of a bar pattern and each separate wire is connected to a drive circuit. A toner dot is printed when two adjacent wires in each layer are driven positively (assuming a negatively charged toner). Toner is then drawn to a hole at the intersection of the two pairs of positively driven wires, passes therebetween and is deposited upon a media sheet.
The Larson system exhibits a number of disadvantages. The array of wires can only be supported by a frame structure around the edge of the print array. Very little sag in the wires can be tolerated due to the tight spacing control which must be maintained between the print wire array and the paper. The array of wires is fragile and each layer must be perfectly insulated from the other, which is difficult considering the number of cross-over points. There also may be some leakage of toner through adjacent holes between wire pairs. Lastly, the holes formed by the intersecting wires are square and may not provide optimum shaped dots for best print resolution.
U.S. Pat. No. 5,121,144 to Larson describes a multiplexing system for a toner projection printer. In lieu of employing a continuous conductive platen behind the media sheet upon which toner is to be deposited, the Larson '144 patent utilizes an insulating platen which includes many conducting wires that are inlaid across the direction of movement of the media sheet. Electrodes which control toner deposition are positioned on an insulating substrate above the media sheet and are connected together in a number of sets, so that only one electrode in each set is directly over a given wire in the conductive platen. Only one platen wire at a time is driven to a high positive voltage (for a negatively charged toner). When an electrode set is also driven positive, the single electrode which resides over the active wire in the platen causes a deposition of toner on the media sheet.
The structure shown in the '144 Larson patent also exhibits a number of disadvantages. The platen structure is complex and includes many precision-inlaid conductors. The insulation between these conductors must withstand a high voltage (e.g., approximately 1000 volts) and must maintain insulating properties, even though it is subject to wear as media sheets pass over it. The drive circuits for the platen wires must also be capable of driving a high voltage--which is a much higher voltage than that required to drive the print electrodes directly (approximately 100 volts). The higher voltage drive circuits are correspondingly more expensive. Finally, the platen with its inlaid wires must be precisely aligned with the printing electrode array to achieve acceptable print quality.
PCT published Application WO 90/14960 to Larson describes an improvement to the electrode structure shown in the Larson '341 patent referred to above. In the PCT published Application, Larson employs isolation electrodes to reduce cross coupling or cross talk between adjacent mesh electrodes. In PCT published Application WO 90/14959 to Larson, a procedure is described for removing deposited toner from an electrode matrix which employs a reverse voltage application during periods between address times. However, when toner particles adhere to the electrode rings, they tend to lose their charge by conduction through the electrode rings. Thus, application of a reverse voltage to remove such particles is ineffective due to their loss of charge.
As can be seen from the above, while toner projection printers eliminate the need for a photoconductor belt or surface, cost and performance improvements are required before the benefits to be obtained by the elimination of the photoconductor component will be realized.
Accordingly, it is an object of this invention to provide an improved toner projection printer which exhibits less toner spreading than heretofore.